Apollo has gotten the better of Cpt. Atom, shrugged off his best energy blasts, and recovered in mere panels from the purported "owning" Atom gave him at the surface of the sun...which required the sun as a plot device and wasn't something Atom could've accomplished normally. I don't think anyone would object that Cpt. Atom is in the same league as Diana, and his arc in Wildstorm is considered canon for both characters.
He also has the moon-clearing feat, showing that his energy output is far superior to anything Diana has done. And he has a few others notable feats, particularly the Reverend Clay fight that was posted last page, as well as the space station destruction....notable because the station had "a docking bay the size of a continent", was probably the size of a small planet altogether, and Apollo wrekcs about a third of it just from crashing into it, and he's fine afterward.
More than anything, it's just annoying to see DC'ers throw one feat out for their person, decide arbitrarily that Wildstorm isn't as good a universe and Apollo is a D-list Supes clone, and blithely assume that Apollo loses. WW isn't the only person this has happened to, and those who are touting Iron Man level people as being good matchups for Apollo obviously just don't know the character.